1. Field of the Invention
The present invention relates to a method for communicating between a control unit and a number of remote units located in the response area of the control unit, for example, a base station and a number of tags in RFID or remote sensor systems, wherein the remote units are prompted to transmit a data sequence to the control unit in executing a command sent by the control unit.
2. Description of the Background Art
Today a wide variety of technical applications are known which use communication systems having at least one control unit or base station and a plurality of remote units arranged in a communication field about the control unit. In this context, the control unit is frequently capable of collecting information contained in the remote units; in addition, it can also be designed to write information in the remote units. The communication field can be composed of physical connections between the control unit and the remote units, or alternatively, can take the form of the transmissions of electromagnetic waves between the control unit and the remote units.
An example of the latter type of systems are radio frequency identification (RFID) systems, in which a control unit in the form of a base station (also known as a reader) transmits signals in the radio frequency range to remote units, which are primarily implemented as integrated circuits with a transmitting and receiving device (antenna) and are called transponders or tags. The tags can be used, in particular, as identification for objects, such as pieces of luggage, or for livestock, or people within the framework of access control. Moreover, when they are suitably designed, such tags can also perform sensor functions, for example temperature measurement, in which case they are typically called remote sensors.
In the aforementioned systems, data transmission from the tag to the base station can either take place with a time offset relative to the data transmission from the base station to the tag—in which case it is called a half-duplex transmission method (for example, Finkenzeller, RFID-Handbuch, 3rd edition (2002), Hanser, p. 40 ff, which was published in English by John Wiley & Sons, and corresponds to page 40 ff of the published English version—or it takes place in both directions at once, which is called full-duplex. For data transmission from the tag to the base station (return link), a so-called backscatter method is frequently used in this context, wherein the tag appropriately modulates and reflects back to the base station a carrier signal (carrier wave) of the base station, which in so-called passive tags can also serve to supply energy. A common modulation method is, for example, amplitude shift keying (ASK).
Within the scope of most of the above-mentioned applications, multiple remote units are present in the response area of the control unit, with the result that a transmission from the control unit is received by more than one remote unit. Hence an information query from the control unit can under certain circumstances result in a plurality of (simultaneous) transmissions from remote units to the control unit (known as multiple access), which generally disrupts or at least impedes reception by the control unit, especially when the remote units transmit with only very low useful signal strength, such as is the case with backscatter-based RFID systems.
To avoid these problems, a variety of anticollision or arbitration methods are known in RFID systems, the principles of which are explained in Finkenzeller, op. cit., pp 203 ff, which corresponds to pp 200 ff of the published English version and which is incorporated by reference herein. Such methods serve to permit the base station to sequentially select individual tags from among a plurality of tags and communicate selectively therewith. Upon conclusion of a communication with a tag (or a group of tags), the tag or tags are often muted until all tags have been addressed in this way without the aforementioned collision problems.
Another fundamental problem in the aforementioned communication systems is associated with the time period that must be calculated for reading information with a plurality of remote units, wherein efforts are frequently made to minimize the length of this time period.
A method is known from, for example, EP 1 172 755 A1, which corresponds to U.S. Publication No. 2002/0024422, and which is directed to reading information in a case of a plurality of transponders of an electronic identification system are operated in a half-duplex mode, in which a base station, upon successfully detecting a first part of a signal sequence transmitted by one of the transponders, transmits a control signal in the form of a notch signal (modulation dip, field gap) which mutes all transponders that are still inactive at that point in time, whereupon the base station uses an additional control signal to cause the transponder in question to transmit the remainder of the sequence. It must be viewed as a particular disadvantage here that an ASK-based backscatter transponder in particular must disable its RSSI channel (RSSI=receiver signal strength indicator) when operating its ASK modulator on account of overshoot problems caused by the occurrence of peaks during ASK modulation. Moreover it is necessary, especially with large transmission distances, to operate with a large modulation index m close to the value m=1 (known as on-off keying) so that the transponder then can no longer reliably determine whether the aforementioned notch signal comes from the base station or whether it was generated by the transponder itself to modulate the carrier signal.
Another disadvantage of the abovementioned, prior art solution consists in that the base station in any case destroys information by sending the notch signal, so that the described method can only be used in a reasonable manner with a modulation index m<1, since a backscatter signal can also occur during a notch here; however, this adversely affects the achievable range in consideration of the foregoing.
Also, in a conventional communication method, each of a number of remote units in the form of transponders first transmits certain header data to a control unit (base station) after a randomly determined time has elapsed, as is taught in, for example, U.S. Pat. No. 6,104,279. When the control unit receives such header data error-free, it can transmit a confirmation signal, which the relevant transponder interprets as a command to continue transmission, while the other transponders are automatically and simultaneously muted.
However, in the conventional art, there may be a statistically possible event of simultaneous transmission of header data by more than one transponder prior to reception of the confirmation signal, thus interference between the header data transmissions can occur, so that the process is delayed by the period of time corresponding to the length of the header data and the random period of time after which the next transponder transmits its header data.